The present invention relates generally to a resonant loop current disturbing probe and more particularly to the measurement of the distribution of the current along an antenna, particularly in the so-called modulated return radiation method.
The modulated return method is shown in FIG. 1.
An antenna 1 is supplied by a generator 2 across a circulator 3 (or hybrid T or coupler). The operating frequency is fo. A small probe 4, formed from one or more conductive turns closed on a photoresistor is moved along the antenna. The probe photoresistor is illuminated by light radiation guided by an optical fibre 5 supplied by a light-emitting diode 6. The latter is controlled by a circuit 7 comprising an oscillator at a frequency fm. The light radiation amplitude-modulated in this way at frequency fm leads to a variation in the resistance of the photoresistor inserted in the probe. The probe is couple to the magnetic field produced by the current flowing in the antenna, so that the electric signal reflected by the antenna is amplitude-modulated at frequency fm. Thus, the spectrum of the reflected signal is formed by a carrier at frequency fo and by two side bands at frequencies fo-fm and fo+fm. Such a signal carries amplitude and phase informations relative to the current flowing in the antenna to the right of the probe. A circuit 100 makes it possible to read these informations.
For example, such a method is described in the article by K. Itzuka entitled "Photoconductive Probe For Measuring Electromagnetic Fields" published in Proceedings of the IEE, vol 110, No. 10, October 1963.
The attached FIG. 2 shows in greater detail the structure of probe 4, where it is possible to see a loop 10, a photoresistor 12 and the optical fibre 5 referred to hereinbefore. Although being satisfactory in certain respects, such a probe lacks sensitivity.